1. Field of the Invention
The present invention relates to a motor control apparatus and an electric motor using the same, and more particularly, to a motor control apparatus and an electric motor using the same which are suitable for use in a magnetic pole position estimation system for estimating a magnetic pole position of an alternating current (AC) synchronous motor.
2. Description of the Related Art
Generally, since the phase of an AC voltage applied to a synchronous motor is determined by a current magnetic pole position of a rotor, exact pole positions of the rotor are required for controlling the synchronous motor. A torque and speed of the synchronous motor can be controlled by conducting a current control or a voltage control based on the detected magnetic pole position. In recent years, a magnetic pole position sensor-less control system has been proposed for controlling a synchronous motor without using a position detector for detecting the magnetic pole position. Since a synchronous motor control apparatus based on the magnetic pole position sensor-less control system eliminates a position detector, the control apparatus could experience a problem of out-of-synchronism due to erroneous detection of magnetic pole position.
To solve this problem, for example, as described in JP-A-9-294390, a conventionally known synchronous motor control apparatus sets a determination level for an effective current value of a stator winding, and detects the out-of-synchronism when the effective current value of the stator winding exceeds the determination level, and a power factor angle between a voltage applied to the stator winding and a stator winding current reaches a value close to 90-.
Also, as described in JP-A-11-55994, a known synchronous motor control apparatus detects a phase difference between an output voltage and an induced voltage of a synchronous motor, calculates a difference between the frequency of an output voltage to the synchronous motor and the rotational speed of the synchronous motor from a changing rate of the detected phase difference value, and controls or changes the frequency of the output voltage to the synchronous motor by a frequency proportional to the calculated difference to prevent the out-of-synchronism.
However, the system described in JP-A-9-294390 requires complicated operations involved in calculating a square root for finding the effective current value of the stator winding of the motor and in calculating the power factor angle.
The control apparatus described in JP-A-11-55994, in turn, is applied to a system for estimating a magnetic pole position using an induced voltage generated by a motor, so that this control apparatus cannot be used in a carrier synchronized position estimating method which estimates a magnetic pole position of a rotor based on a current of the motor detected in synchronism with a carrier of a PWM signal.